Method, system and computer program of visualizing the surface texture of the wall of an internal hollow organ of a subject based on a volumetric scan thereof

ABSTRACT

The invention concerns a method of visualising an internal hollow organ ( 1 ) of a subject based on a volumetric scan thereof. A three-dimensional image of the internal hollow organ is reconstructed in which a layer ( 2 ) of a predetermined depth (d) in at least part of the wall surface is defined. Property values associated with the segments of the layer are determined to which visualisation parameters are assigned. The visualisation parameters are added to the three-dimensional display in order to show the wall structure of the internal hollow organ as a texture map. The invention also refers to a system for visualising an internal hollow organ of a subject based on a volumetric scan thereof, which system comprises means for carrying out the steps of the method according to the invention. The invention also relates to a computer program to carry out the method according to the invention.

The present invention relates to a method of visualising an internalhollow organ of a subject based on a volumetric scan thereof, saidmethod comprising the step of:

-   -   a)Reconstructing a three-dimensional image of the internal        surface of the hollow organ.

Such a method is known in the art and forms the basis for a number ofcomputer programs designed by different experts in the field providing atechnique called “virtual endoscopy”. Based on a volumetric scan of apatient, for instance generated by means of Computed Tomography, a datamodel is created from which three-dimensional endoscopic images arereconstructed by means of known three-dimensional reconstructiontechniques. These 3D endoscopic images provide a view as seen from avantage point that lies within the hollow organ close to the internalsurface thereof. Such computer programs offer a medically skilled personan opportunity to examine the internal organs of the patient without theneed for invasive examination like true endoscopy. The thusreconstructed 3D endoscopic images can for instance be evaluated on acomputer by a medically skilled person for diagnosis.

The known method has the disadvantage that although the resulting 3Dimages are a true representation of the shape of the internal surface ofthe hollow organ, the texture is missing. Said texture generally mayreveal important additional information about the structural detail ofthe surface, such as the vascularisation pattern. The lack of texture isan important reason why physicians still tend to choose truly invasiveexamination over virtual examination.

It is an object of the method according to the invention to provide amethod of the type as described above that can visualise certainproperties of the surface as a texture on the surface of the internalhollow organ.

The method according to the invention is therefore characterised in thatthe method further comprises the steps of:

-   -   b) Defining a layer of a predetermined depth in at least part of        the wall surface of the hollow organ;    -   c) Determining property values associated with the segments of        the layer;    -   d) Assigning visualisation parameters to the property values;        and    -   e) Adding the visualisation parameters to the three-dimensional        image as a texture map in order to show the wall structure of        the internal hollow organ.

The method according to the invention thus can visualise thevascularisation pattern as part of the texture of the surface of thehollow organ. Changes in the vascularisation pattern allow to make adistinction between different types of abnormalities, such as polyps orretained stool, and even to differentiate between benign and malignantabnormalities, the details of which are referred to in the preferredembodiments as part of the sub claims.

In a first preferred embodiment of the method according to the inventionstep c) comprises the step of: Determining the maximum intensity valuefor each group of segments in a direction essentially perpendicular tothe internal surface of the hollow organ. By determining the maximumintensity the details of malignant abnormalities become clearly visibleas the associated tissue usually shows higher intensity values, due to ahigher concentration of blood vessels.

According to another preferred embodiment of the method according to theinvention step c) further comprises the step of: Determining the minimumintensity value for each group of segments in a direction essentiallyperpendicular to the internal surface of the hollow organ. The minimumintensity value provides additional information about areas having lowerintensity values, such as air, which may indicate the presence ofretained stool or a loop in the colon. The use of this additionalinformation may aid in preventing misdiagnosis.

In yet another preferred embodiment the method step d) further comprisesthe step of: Assigning colour values to the property values according toa predetermined colour scheme. By choosing the real colours associatedwith the tissue a natural impression can be given of the texture of thesurface.

Preferably step e) further comprises the step of: Superimposing thecolour values to the three-dimensional image in order to show the wallstructure of the internal hollow organ. In a fairly efficient way thetexture can thus be integrated in the existing 3D model.

According to a refined embodiment of the method step b) furthercomprises the step of: Defining a layer of a predetermined depthessentially corresponding to the depth of the mucosa on the wall surfaceof the internal hollow organ. This embodiment is especially developedfor use with internal hollow organs that are coated with mucosa, such asthe colon or the trachea. The blood vessels of the mucosa provide allrelevant information about the texture of the surface.

Interesting property values comprise density values or thickness valuesof the layer in general, and more specific of the mucosa.

The invention further relates to a system for visualising an internalhollow organ of a subject based on a volumetric scan thereof, whichsystems comprises means for carrying out the steps of the methodaccording to the invention.

The invention also concerns a computer program to carry out the methodaccording to the invention.

The invention will be further explained by means of the attacheddrawing, in which:

FIG. 1 shows a flow diagram presenting an overview of the steps of themethod according to the invention; and

FIG. 2 schematically shows a cross section through the colon wall as anillustration of step 20 of the method according to the invention.

In general the method according to the invention refers to virtualtechniques for examination of a subject, which is usually a humanpatient, but can also for instance be an animal. Said techniques allowan inner view of hollow structures of the subject, e.g. organs, bloodvessels, etc., by means of computer graphics. A virtual camera is placedin a three-dimensional data volume representing (part of) the subject.The method according to the invention will now be described according toa preferred embodiment, which relates to virtual endoscopy performed ona human patient.

In order to require the 3D patient data several known medicalexamination techniques can be used, such as Computed Tomography (CT) orMagnetic Resonance Tomography (MR). The 3D data are visualised by meansof known three-dimensional reconstruction techniques. For this purposedifferent suitable volume rendering techniques are known in the field ofcomputer graphics. Preferably use is made of iso-surface volumerendering techniques, which are for instance described in the article“Iso-surface volume rendering”, by M. K. et al., Proc. of SPIE MedicalImaging '98, vol. 3335, pp 10-19. Thus a virtual environment is createdthat simulates endoscopy.

The method comprises the following technical steps:

Step 10: Reconstructing a three-dimensional image of the internalsurface of the hollow organ.

A variety of visualisation techniques are available to the personskilled in the art to simulate a three-dimensional view of the colon.Several examples include:

-   -   a) the “view point” technique also referred to as virtual        endoscopy, wherein a user navigates through the colon;    -   b) the “unfolded cube” technique, wherein the colon wall is        projected onto the walls of a cube, which is next unfolded to        provide a natural view of the colon; and    -   c) the “stretched path” technique, wherein the colon wall is        projected onto the walls of a cylinder, which is next unfolded        and stretched.

The view point technique is a classical technique that is known in theart and among others described by Rogalla P, Terwisscha van ScheltingaJ, Hamm B (Eds) in “Virtual endoscopy and related 3D techniques”,Berlin, Springer Verlag (2001). This book is part of the series MedicalRadiology Diagnostic Imaging edited by: Baert A L, Sartor K, en Youker JE. The unfolded cube technique is in more detail described in thearticle “Quicktime VR- an image based approach to virtual environmentnavigation”, by S. E. Chen, SIGGRAPH 95, held on 6-11 August 1995, LosAngeles, Calif., USA, Conference Proceedings, Annual Conference Series,pages 29-38. The stretched path technique is in more detail described inthe following article by D. S. Paik, C. F. Beaulieu, R. B. Jeffrey, Jr.,C. A. Karadi, S. Napel, “Visualization Modes for CT Colonography usingCylindrical and Planar Map Projections.” J Comput Assist Tomogr 24(2),pages 179-188, 2000.

All techniques result in a segmentation of the colon based on a voxelmodel comprising the data of a volumetric scan that is projected on aflat surface and represented as a surface model.

Step 20: Defining a layer of a predetermined depth in at least part ofthe wall surface of the internal hollow organ.

This step is illustrated by means of FIG. 2 showing a cross sectionthrough the colon 1. In order to define such a layer 2 the two surfaces3, 4 defining the boundaries of the layer 2 need to be defined. Dilationprocedures known in the art can be used for this purpose and are forinstance described by Giardina CR and Daugherty ER in “Morphologicalmethods in image processing”, Upper Saddle River N.J., U.S.A., PrenticeHall (1988). Surface 3 starts preferably on or slightly after theair-tissue transition depending on the technique used.

When the internal hollow organ is coated with mucosa, as is the casewith the colon or trachea, the depth (d) of the layer is preferablydefined essentially equal to the depth of the mucosa, which generallylies between 2 and 4 mm for the colon.

Step 30: Determining property values associated with the voxels in thelayer.

Many interesting property values can be thought of, such as densityvalues or thickness values. In order to determine the density valuespreferably a technique known in the art as Maximum Intensity Projection(MIP) is used. For a detailed description of this technique reference ismade to the article “A fast progressive method of maximum intensityprojection” by Kim K. H. and Park H. W., published in Comput. Med.Imaging Graph. 2001, September-October;25(5):pages 433-441.

Herein the maximum intensity value for each group of voxels in adirection essentially perpendicular to the surface of the internalhollow organ is determined. A number of normal vectors (n) are shown inFIG. 2 illustrating the direction perpendicular to the surface wall. Thedirection of these vectors can be established based on known techniques,such as surface rendering techniques, one of which is for instancedescribed in the article “Iso-surface volume rendering”, by M. K. etal., Proc. of SPIE Medical Imaging '98, vol.3335, pp 10-19. Thedirection can also be found by an algorithm known in the art using agradient of Hounsfield numbers of the tissue that is for instancedescribed by Hoehne K H, Bernstein R, “Shading 3D images from CT usinggrey-level gradients”, IEEE Transactions on Medical Imaging, Vol. 5, Nr1 (1986), pages 45-57. In short according to this algorithm thedirection of the maximum gradient is determined in sub voluminacomprising a number of voxels. The voxel lying in the centre of such asub volume needs to lie at the segmentation surface. The direction ofthe maximum gradient found is set equal to the direction of the normalto the surface. This normal vector can be found for each voxel formingpart of the segmentation surface.

Preferably the group of voxels for which the (maximum) intensity valueis determined, as mentioned above, includes all voxels the centre ofwhich lies in a predetermined sub volume. To define each sub volume animaginary line is drawn in the produced part of a normal vectorpenetrating the tissue. Part of the dimensions of the sub volume isdefined depending on the resolution of the data. As an example the subvolume may have a width of approximately one voxel, preferably half avoxel on each side of the normal vector. The depth of the sub volumewill generally be defined by the depth of the layer.

In case of an MIP surface 3 starts on the air-tissue transition.Preferably an MIP is determined for all normal vectors in the layer.Depending on the application and the users wishes the layer may coverthe entire internal wall of the object under examination or a selectedpart of it.

A malignant abnormality, such as a tumour, will result in higherintensity values compared to those of the surrounding tissue and can nowbe easily distinguished.

Additionally a technique known in the art as Minimum IntensityProjection (mIP) may be used. This technique is described in the article“Three-dimensional spiral CT cholangiography with minimum intensityprojection in patients with suspected obstructive biliary disease:comparison with percutaneous transhepatic cholangiography” by Park S J,Han J K, Kim T K and Choi B I, published in Abdom. Imaging. 2001,May-June; 26(3) pages 281-286. Herein the minimum intensity value foreach group of voxels in a direction essentially perpendicular to thesurface of the internal hollow organ is determined. With respect to allother details the procedure is analogous to the procedure describedabove for MIP. Application of the mIP provides additional informationabout benign abnormalities found in the wall structure of the object.For instance, contamination, such as retained stool, may be present inthe colon. The mIP will signal this by presenting a very low intensityvalue at the location of the contamination due to the presence of airbubbles and the lack of contrast medium therein. The organ may alsocontain loops, which may lead to erroneous information in case the layer2 inadvertently comprises more than just the intended mucosa at onelocation. This situation will also be signalled by the mIP presenting avery low intensity value at the location of the loops. As the locationof the loops usually will be significantly larger than the location ofthe contamination, a distinction can be made by taking into account thesize of the abnormality as well. In case of an mIP surface 3 startsslightly after the air-tissue transition. Preferably a margincorresponding to the width of the spatial resolution (typically half aslice in case of CT data) is used.

As an alternative to the density values visualised as described aboveother property values may be visualised, such as thickness values of thelayer 2. To this end a number of the above described techniques can alsobe used. In addition thereto the border between the layer 2 and thelayer behind it should be established. In the example described whereinlayer 2 is the mucosa layer, the layer behind it usually is a layer offat. The border between these layers can for instance easily bedetermined by determining the Hounsfield number, which differs greatlyfor mucosa and fat tissue.

Step 40: Assigning visualisation parameters to the property values.

In order to make the variation in property values found in the surfaceof the inner wall of the organ visible, different visualisationparameters are assigned to corresponding different property valuesaccording to a predetermined scheme. Preferably colour values areassigned to the property values according to a predetermined colourscheme, such as a colour look-up table.

A suitable colour scheme for visualisation of the density of the colonmay range from yellow (f.i. when the intensity value=0) to (dark) redfor higher intensity values. A suitable colour scheme for the density ofthe trachea may range from pink (f.i. when the intensity value=0) to(dark) red for higher intensity values.

The thickness of the (mucosa) layer can be visualised using any suitablecolours. An example may be red for normal thickness and darker colours,such as green or blue, for thicker areas. The thinner areas may berepresented in lighter colours, such as orange or yellow.

It is noted that many other suitable visualisation parameters will beapparent to a person skilled in the art, such as grey values,patternising values etc.

Step 50: Adding the visualisation parameters to the three-dimensionalimage as a texture map in order to show the wall structure of theinternal hollow organ.

Finally the visualisation parameters need to be incorporated in thethree-dimensional image in order to show the wall structure of theinternal hollow organ. Preferably the parameter values are superimposedonto the three-dimensional image thus revealing more surface details.

The method according to the invention is preferably carried out by asystem for visualising an internal hollow organ of a subject based on avolumetric scan thereof, which systems comprises means for carrying outthe steps of the method according to the invention. Said meanspreferably comprise a computer program. Based on the explanation givenherein a skilled person will be able to translate the steps of themethod into such a computer program to carry out the method.

The system described can be directly coupled to the data acquisitionsystem for acquiring the data of the subject concerned. This data setcan be acquired by means of various techniques, such as 3D X-rayrotational angiography, computed tomography, magnetic resonance imagingor magnetic resonance angiography. When the method according to theinvention is applied to a human patient, the patient preferably isadministered a contrast agent suitable for medical use. The type ofcontrast agent depends on the application and can for instance be anintravenous contrast agent to aid in distinguishing the blood vessels onthe inner surface wall of the colon or trachea.

Summarising the invention refers to a post-processing method forvisualising variations in property values, such as density or thicknessof the inner surface wall of hollow objects in order to reveal moredetail thereof. The method is especially developed to increase theaccuracy of patient diagnosis. Application of this method in combinationwith known virtual visualisation methods, such as virtual endoscopy,results in a virtual image yielding the same information ascorresponding invasive medical examination methods, such as colonoscopyand bronchioscopy.

The invention is of course not limited to the described or shownembodiment. The method may be used to visualise surface details of othermedical objects, such as blood vessels or trachea, and may even be usedoutside the field of medicine. The invention therefore generally extendsto any embodiment, which falls within the scope of the appended claimsas seen in light of the foregoing description and drawings.

1. A method of visualising an internal hollow organ of a subject basedon a volumetric scan thereof, said method comprising the step of: a)Reconstructing a three-dimensional image of the internal surface of thehollow organ; Characterised in that the method further comprises thesteps of: b) Defining a layer of a predetermined depth (d) in at leastpart of the wall surface of the hollow organ; c) Determining propertyvalues associated with the segments of the layer; d) Assigningvisualisation parameters to the property values; and e) Adding thevisualisation parameters to the three-dimensional image as a texture mapin order to show the wall structure of the internal hollow organ.
 2. Amethod according to claim 1, wherein step c) comprises the step of: (i)determining the maximum intensity value for each group of segments in adirection (n) essentially perpendicular to the internal surface of thehollow organ.
 3. A method according to claim 10, wherein step c) furthercomprises the step of: (i) Determining the minimum intensity value foreach group of segments in a direction (n) essentially perpendicular tothe internal surface of the hollow organ.
 4. A method according to claim1, 2 or 3, wherein step d) further comprises the step of: (i) Assigningcolour values to the property values according to a predetermined colourscheme.
 5. A method according to claim 4, wherein step e) furthercomprises the step of: (i) Superimposing the colour values to thethree-dimensional image in order to show the wall structure of theinternal hollow organ.
 6. A method according to claim 1, wherein step b)further comprises the step of: (i) Defining a layer of a predetermineddepth (d) essentially corresponding to the depth of the mucosa on thewall surface of the internal hollow organ.
 7. A method according toclaim 1, wherein the property values comprise density values of thelayer.
 8. A method according to claim 1, wherein the property valuescomprise thickness values of the layer.
 9. A system for visualising aninternal hollow organ of a subject based on a volumetric scan thereof,which systems comprises means for carrying out the steps of the methodaccording to claim
 1. 10. Computer program to carry out the methodaccording to claim 1.